1. Field of the Invention
The present invention relates to noise reduction in digital communication systems.
2. Related Art
In many communication systems, preventing interference between transmitted signals and received signals is essential. An example system is the Asymmetrical Digital Subscriber Line (ADSL) system. ADSL is a high-speed full duplex broadband transmission technique to connect a modem to the internet over ordinary telephone lines. For all ADSL applications, a hybrid circuit is used to couple receive (Rx) signals and transmit (Tx) signals from the telephone line. The hybrid circuit is also used to separate or decouple the Tx and Rx signals from each other.
Under ideal circumstances, the hybrid circuit is able to completely decouple the Tx and Rx signals from one another, so that the signal being transmitted does not interfere with the signal being received. However, due to imperfections in, for example, a telephone line and limited performance of the hybrid circuit, the hybrid circuit will not be able to completely decouple the Tx and Rx signals. Therefore, some amount of the residual Tx signal and Tx noise will be coupled onto the incoming Rx signal.
In order for the Rx signal to be correctly received, the interference from this Tx-to-Rx coupling must be minimized. This problem is exacerbated because the Rx signal is very small when compared to the Tx signal, and any residual Tx signal or noise can corrupt the Rx signal integrity.
As mentioned above, ADSL is only one type of communications system that experiences this problem. In many other communication systems, Tx noise needs to be minimized for similar reasons. In general, Tx noise must be limited so that the noise generated by a transmitter of a specific user does not interfere with the receiver of that same user (as in the case of ADSL), another user utilizing the same communications system, or another user using an entirely different communications system.
Although techniques exist for removing the residual Tx signal (e.g., echo cancellation), there are no comparable techniques to remove the residual Tx noise. Therefore, what is needed is a method and system for keeping Tx noise at a minimum.